U.S. patent application number 12/275576 was filed with the patent office on 2009-06-04 for glass substrate for magnetic disk and magnetic disk apparatus.
This patent application is currently assigned to THE FURUKAWA ELECTRIC CO., LTD.. Invention is credited to Yasuhiro NAKA, Nobuaki ORITA.
Application Number | 20090142626 12/275576 |
Document ID | / |
Family ID | 40676047 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090142626 |
Kind Code |
A1 |
ORITA; Nobuaki ; et
al. |
June 4, 2009 |
GLASS SUBSTRATE FOR MAGNETIC DISK AND MAGNETIC DISK APPARATUS
Abstract
A glass substrate for a magnetic disk satisfies Ra1.ltoreq.0.8
[nm], 0 [nm].ltoreq.Ra1-Ra2.ltoreq.0.2 [nm], Wa1.ltoreq.0.6 [nm],
and 0 [nm].ltoreq.Wa2-Wa1.ltoreq.0.2 [nm]. Ra1 is an average
surface roughness of a first annular area between 1 mm and 3 mm
outward from an inner periphery of a main surface of the glass
substrate, Ra2 is an average surface roughness of a second annular
area between 1 mm and 3 mm inward from an outer periphery of the
main surface, Wa1 is an average waviness of the first area in a
circumferential direction of the glass substrate having a cycle of
300 .mu.m to 5 mm, and Wa2 is an average waviness of the second
area in the circumferential direction having a cycle of 300 .mu.m
to 5 mm.
Inventors: |
ORITA; Nobuaki; (Tokyo,
JP) ; NAKA; Yasuhiro; (Tokyo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
THE FURUKAWA ELECTRIC CO.,
LTD.
Tokyo
JP
|
Family ID: |
40676047 |
Appl. No.: |
12/275576 |
Filed: |
November 21, 2008 |
Current U.S.
Class: |
428/846.9 |
Current CPC
Class: |
C03C 19/00 20130101;
G11B 5/73921 20190501; G11B 5/82 20130101 |
Class at
Publication: |
428/846.9 |
International
Class: |
G11B 5/62 20060101
G11B005/62 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2007 |
JP |
2007-309395 |
Claims
1. A glass substrate for a magnetic disk that is mounted on a hard
disk drive, wherein the glass substrate satisfies Ra1.ltoreq.0.8
nanometer, 0 nanometer<Ra1-Ra2.ltoreq.0.2 nanometer,
Wa1.ltoreq.0.6 nanometer, and 0 nanometer<Wa2-Wa1.ltoreq.0.2
nanometer where Ra1 is an average surface roughness of a first
annular area between 1 millimeter and 3 millimeters outward from an
inner periphery of a main surface of the glass substrate, Ra2 is an
average surface roughness of a second annular area between 1
millimeter and 3 millimeters inward from an outer periphery of the
main surface, Wa1 is an average waviness of the first area in a
circumferential direction of the glass substrate having a cycle of
300 micrometers to 5 millimeters, and Wa2 is an average waviness of
the second area in the circumferential direction having a cycle of
300 micrometers to 5 millimeters.
2. The glass substrate according to claim 1, wherein the glass
substrate is mountable on a hard disk drive of size 1-inch or a
hard disk drive that employs a magnetic disk that is smaller than
that employed in a hard disk drive of size 1-inch.
3. A magnetic disk apparatus that operates based on a load/unload
method and includes a magnetic disk made of a glass substrate,
wherein the magnetic disk satisfies Ra1.ltoreq.0.8 nanometer, 0
nanometer<Ra1-Ra2.ltoreq.0.2 nanometer, Wa1.ltoreq.0.6
nanometer, and 0 nanometer<Wa2-Wa1.ltoreq.0.2 nanometer where
Ra1 is an average surface roughness of a first annular area between
1 millimeter and 3 millimeters outward from an inner periphery of a
main surface of the glass substrate, Ra2 is an average surface
roughness of a second annular area between 1 millimeter and 3
millimeters inward from an outer periphery of the main surface, Wa1
is an average waviness of the first area in a circumferential
direction of the glass substrate having a cycle of 300 micrometers
to 5 millimeters, and Wa2 is an average waviness of the second area
in the circumferential direction having a cycle of 300 micrometers
to 5 millimeters.
4. The magnetic disk apparatus according to claim 3, wherein the
magnetic disk is a hard disk drive of size 1-inch, or a hard disk
drive that employs a magnetic disk that is smaller than that
employed in a hard disk drive of size 1-inch.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a glass substrate for a
magnetic disk and a magnetic disk apparatus.
[0003] 2. Description of the Related Art
[0004] Nowadays, drastic technical innovations are needed in
information recording technologies, especially in magnetic
recording technologies as information technologies (IT) evolve. To
catch up the trend, fast-paced increase in information recording
density continues for a magnetic disk that is mounted on a hard
disk drive (HDD) functioning as a magnetic disk apparatus that
serves as a computer storage.
[0005] Recently, there is an increasing demand for mounting an HDD
in a portable device. To satisfy such a demand, a glass substrate
with high strength, high rigidity, and high impact-resistance is
employed as a substrate for a magnetic disk. A glass substrate can
easily provide a smooth surface, so that a fly height of a magnetic
head that performs recording and reproducing while flying over a
magnetic disk can be reduced. Therefore, a magnetic desk with
higher information recording density is attainable by the use of
the glass substrate as a magnetic disk substrate. Namely, the glass
substrate is advantageous in that it reduces the fly height of the
magnetic head.
[0006] To increase the information recording capacity in the
magnetic disk, an information signal-unrecorded area of the
magnetic disk needs to be reduced. In achieving this, introduction
of a load/unload method (an LUL method) otherwise known as "a ramp
load method" that enables increasing the information recording
capacity has been promoted in place of a conventional contact start
stop method (a CSS method) as a start/stop method for an HDD.
[0007] The conventional CSS method disadvantageously needs to have
a CSS zone in a magnetic disk, on which a magnetic head is placed
when the magnetic disk is not in use (in a stop state).
[0008] By contrast, in the LUL method, the magnetic head moves
toward an outer periphery of the magnetic disk and then it stops at
a position that is out of a space over the magnetic disk when the
magnetic disk is not in use, thus enabling to prevent contact of
the magnetic head with the magnetic disk unlike the CSS method.
This can eliminate the need to provide antistick concavities and
convexities on the surface of the magnetic disk that have been
generally provided in the CSS zone. By the LUL method, a highly
smooth main surface of the magnetic disk is attainable.
[0009] Thus, further reduction of the fly height of the magnetic
head can be realized by the LUL magnetic disk compared with the CSS
magnetic disk, so that a signal noise ratio (an S/N ratio) of
recording signals can be improved and higher recording density is
attainable.
[0010] The introduction of the LUL method has enabled a narrower
fly height for the magnetic head; however, this created another
requirement. That is, a stable operation of the magnetic head is
required at a nanosized fly height of equal to or less than 10
nanometers. In such a minute space, fly stiction phenomenon
frequently occurs when the magnetic head flies over the magnetic
disk.
[0011] The fly stiction phenomenon is an unstable fly height or
flying state of a magnetic head that is flying over a magnetic
disk, thus, generating irregular reproduction output fluctuation.
This fly stiction phenomenon may cause head crash, that is, bumping
the magnetic disk by the flying magnetic head occurs.
[0012] Efforts have been made for the conventional HDDs to prevent
such fly stiction phenomenon by applying a higher rotation velocity
to the magnetic disk, thus applying a higher relative linear
velocity between the magnetic disk and the magnetic head, and by
stabilizing the fly height or flying state by the structure of the
magnetic head.
[0013] However, recently, demand has been increasing for a smaller
HDD mountable on devices such as a cell phone, a digital camera, a
portable information device, and a car navigation system that have
a much smaller device size than a personal computer (PC) and need a
high response speed. For example, the small-sized HDD that
accommodates a magnetic disk manufactured using a substrate with an
outer diameter of equal to or less than 50 millimeters and a
thickness of equal to or less than 0.5 millimeter is needed.
[0014] A small magnetic disk of which outer diameter is equal to or
less than 50 millimeters is typically used for a small HDD. In the
small magnetic disk, an outer circumference and an inner
circumference are small, so that the relative linear velocity
between the magnetic disk and the magnetic head is low. In
addition, a small spindle motor is generally used to rotate the
small magnetic disk. Because the spindle motor is small, further
speeding-up of the rotation of the magnetic disk is not easy, which
may influence the stability of the fly height and flying state of
the magnetic head, or may not sufficiently prevent occurrence of
the fly stiction.
[0015] A small magnetic head is used in the small magnetic disk.
The stability of the fly height or flying state of the magnetic
head is low.
[0016] In addressing prevention of the fly stiction phenomenon, for
example, Japanese Patent Application Laid-open No. 2005-317181
discloses to increase the surface roughness in a radial direction
of a main surface of a circular substrate by forming an anisotropic
texture on the main surface in the generally circumferential
direction. On the other hand, Japanese Patent Application Laid-open
No. 2007-12157 teaches to use a disk substrate having a diameter of
equal to or less than 1 inch and having a relation represented by 0
nanometer<Ra1-Ra2.ltoreq.0.2 nanometer, where Ra1 is an average
surface roughness of an inner circumferential surface of data area
and Ra2 is an average surface roughness of an outer circumferential
surface of the data area.
[0017] As stated above, the technologies in Japanese Patent
Application Laid-open No. 2005-317181 and Japanese Patent
Application Laid-open No. 2007-12157 have addressed the stable
floating characteristics of the magnetic head by increasing the
average surface roughness of a glass substrate for a magnetic disk
in a circumferential direction from an outer circumferential side
to an inner circumferential side of a main surface of the glass
substrate. The arithmetic average roughness means the
arithmetically averaged roughness of the surface of the glass
substrate for the magnetic disk measured by causing a measuring
probe to scan the glass substrate in the circumferential direction
when a 5-micrometer.times.5-micrometer area of the glass substrate
is measured by an atomic force microscope.
[0018] However, there has been a problem that fly stiction
phenomenon in a further downsized magnetic disk could not be surely
prevented only by the conventional controlling of the average
surface roughness in the circumferential direction of the
substrate.
[0019] In particular, in a recent magnetic disk with a fly height
of less than 10 nanometers, air molecule is in the order of a mean
free path (64 nanometers) of a typical air molecule. Thus, the
generation of a floating pressure can not be explained by the flow
of an airflow continuum. The influence of collision of air
molecules with a solid wall is larger than that of a viscosity
resistance generated due to collision between air molecules and air
molecules.
SUMMARY OF THE INVENTION
[0020] It is an object of the present invention to at least
partially solve the problems in the conventional technology.
[0021] According to an aspect of the present invention, there is
provided a glass substrate for a magnetic disk that is mounted on a
hard disk drive. The glass substrate satisfies Ra1.ltoreq.0.8
nanometer, 0 nanometer<Ra1-Ra2.ltoreq.0.2 nanometer,
Wa1.ltoreq.0.6 nanometer, and 0 nanometer<Wa2-Wa1.ltoreq.0.2
nanometer, where Ra1 is an average surface roughness of a first
annular area between 1 millimeter and 3 millimeters outward from an
inner periphery of a main surface of the glass substrate, Ra2 is an
average surface roughness of a second annular area between 1
millimeter and 3 millimeters inward from an outer periphery of the
main surface, Wa1 is an average waviness of the first area in a
circumferential direction of the glass substrate having a cycle of
300 micrometers to 5 millimeters, and Wa2 is an average waviness of
the second area in the circumferential direction having a cycle of
300 micrometers to 5 millimeters.
[0022] According to another aspect of the present invention, there
is provided a magnetic disk apparatus that operates based on a
load/unload method and includes a magnetic disk made of a glass
substrate. The magnetic disk satisfies Ra1.ltoreq.0.8 nanometer, 0
nanometer<Ra1-Ra2.ltoreq.0.2 nanometer, Wa1.ltoreq.0.6
nanometer, and 0 nanometer<Wa2-Wa1.ltoreq.0.2 nanometer, where
Ra1 is an average surface roughness of a first annular area between
1 millimeter and 3 millimeters outward from an inner periphery of a
main surface of the glass substrate, Ra2 is an average surface
roughness of a second annular area between 1 millimeter and 3
millimeters inward from an outer periphery of the main surface, Wa1
is an average waviness of the first area in a circumferential
direction of the glass substrate having a cycle of 300 micrometers
to 5 millimeters, and Wa2 is an average waviness of the second area
in the circumferential direction having a cycle of 300 micrometers
to 5 millimeters.
[0023] The above and other objects, features, advantages and
technical and industrial significance of this invention will be
better understood by reading the following detailed description of
presently preferred embodiments of the invention, when considered
in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a plan view of a magnetic disk apparatus according
to an embodiment of the present invention;
[0025] FIG. 2 is a flowchart of a method of manufacturing a glass
substrate for a magnetic disk according to the embodiment;
[0026] FIG. 3 is a schematic diagram illustrating a plan view and a
cross sectional view of the glass substrate;
[0027] FIG. 4 is a side view illustrating part of a polishing
machine that simultaneously polishes front and rear surfaces of the
glass substrate;
[0028] FIG. 5 is a plan view of the polishing machine without an
upper table;
[0029] FIG. 6 is a schematic diagram representing measurement
positions of a surface roughness and a waviness of the glass
substrate;
[0030] FIG. 7 is a table depicting results of surface roughness
measurement of samples;
[0031] FIG. 8 is a table depicting results of waviness measurement
of the samples; and
[0032] FIG. 9 is a table depicting results of measurement of
touchdown (TD) property and takeoff (TO) property of the
samples.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] Exemplary embodiments of the present invention are explained
in detail below with reference to the accompanying drawings.
[0034] Based on the above idea that a fly height is largely
influenced by collision of air molecules with a solid wall, a
research made by the inventors of the present patent application
showed that air molecules travel beyond the order of a floating
space of a magnetic disk and interaction of the air molecules with
a solid wall of a substrate is influenced by the shape
characteristics in wider ranges than conventionally perceived. In
other words, controlling the average surface roughness at a
5-micrometer.times.5-micrometer area of a substrate stated in
Japanese Patent Application Laid-open No. 2007-12157 is not enough
for surely preventing occurrence of fly stiction. Another finding
is that the surface shape characteristics of a substrate in wider
ranges including fluctuation in a circumferential direction of the
substrate are important.
[0035] FIG. 1 is a plan view of a magnetic disk apparatus 100
according to an embodiment of the present invention. The magnetic
disk apparatus 100 employs an LUL method and includes a magnetic
disk 102, an arm 104, and a ramp 107 on a base 101.
[0036] The magnetic disk 102 is mounted on a spindle motor (not
shown) that is positioned under the magnetic disk 102 through a
clamp 103, and rotates and stops by the action of the spindle
motor. The arm 104 is a rotary actuator and rotates around a rotary
shaft 105. A slider S having a magnetic head is mounted near the
tip of the arm 104. A lift tab 106 is provided at the tip of the
arm 104. The ramp 107 is provided above and near an outer periphery
of the magnetic disk 102.
[0037] When the magnetic disk 102 is in a stand still state (i.e.,
not rotating), the arm 104 is positioned such that the slider S
stays away from a main surface (front surface) of the magnetic disk
102 and the lift tab 106 rides on the ramp 107. When the magnetic
disk 102 starts to rotate, the arm 104 rotates around the rotary
shaft 105 counterclockwise, the lift tab 106 slidingly moves on the
ramp 107, and the slider S is loaded on the main surface of the
magnetic disk 102 to be opposed thereto.
[0038] In the magnetic disk 102, a recording zone made of a
magnetic material is formed in a main surface of a glass substrate
1. The glass substrate 1 for producing the magnetic disk 102 is
explained below.
[0039] FIG. 2 is a flowchart of a method for manufacturing the
glass substrate 1. The glass substrate 1 is manufactured through
sequential steps of a redrawing process (Step S101), a shape
machining process (Step S102), an end-surface mirror-polishing
process (Step S103), a main-surface rough-polishing process (Step
S104), and a main-surface precision-polishing process (Step
S105).
[0040] In the redrawing process, a 0.6-millimeter-thick sheet glass
is redrawn from a sheet glass preform made of amorphous
aluminosilicate glass. The surface roughness Ra of the sheet glass
is about 0.8 nanometer. The redrawing process is performed by the
redrawing method disclosed in, for example, Japanese Patent
Application Laid-open No. 2007-126302. The redrawing method
disclosed in Japanese Patent Application Laid-open No. 2007-126302
is preferable because a glass plate having small surface roughness
is easily produced with this redrawing method. However, other known
methods such as a float method, a fusion method, or a down-draw
method can also be employed using a molten glass as a material.
[0041] In the shape machining process, a 0.6-millimeter-thick
disk-shaped glass substrate having a diameter of 28.7 millimeters
is formed from the sheet glass redrawn in the redrawing process.
Thereafter, a circular hole 1a of diameter 6.1 millimeter is formed
in the center of the glass substrate using a cylindrical polishing
stone, the outer peripheral end surface of the glass substrate is
polished so that the glass substrate has a diameter of 27.43
millimeters, and then the outer peripheral end surface and an inner
peripheral end surface are chamfered. As a result, the glass
substrate 1 having the circular hole 1a as shown in FIG. 3 is
produced. The maximum surface roughness Rmax of the end surface of
the glass substrate 1 is about 4 micrometers. Generally, a
65-millimeter-outer-diameter magnetic disk is incorporated in a
2.5-inch HDD.
[0042] In the end-surface mirror-polishing process, the glass
substrate 1 is rotated and the end surfaces (the outer and inner
peripheral end surfaces) of the glass substrate 1 are polished by a
conventional brush polishing method such that the maximum surface
roughness Rmax is made to be about 1 micrometer and the average
surface roughness Ra is made to be about 0.3 micrometer. The
polished main surface of the glass substrate 1 is cleaned with
water. A plurality of the glass substrates 1 are stacked and end
surfaces of those glass substrates 1 are polished in batch in the
end-surface mirror-polishing process. It is preferable that the
polishing of the end surfaces be performed before the main-surface
polishing process to avoid an awkward situation where scratches and
the like remain on the main surface of the glass substrate 1. By
this end-surface mirror-polishing process, the end surfaces of the
glass substrate 1 are polished mirror-like, so that generation of
particles and the like is preventable. The diameter of the polished
glass substrate 1 is 27.4 millimeters.
[0043] In the main-surface rough-polishing process, the main
surface is roughly polished using a polishing machine 2 including a
planetary gear mechanism that simultaneously rough-polishes the
main surface and a rear surface. FIG. 4 is a side view illustrating
part of the polishing machine 2. The polishing machine 2 includes
an upper table 3 and a lower table 4 both made of cast iron,
carriers 6 arranged between the upper table 3 and the lower table 4
to retain the glass substrates 1, and polishing stones 5 made of
cerium oxide, each of which is arranged on the upper table 3 and
the lower table 4 to be in contact with the glass substrates 1.
Namely, the polishing machine 2 retains the glass substrates 1 with
the carriers 6 between the upper table 3 and the lower table 4,
presses the glass substrates 1 by the upper table 3 and the lower
table 4 at a predetermined processing force, and rotates the upper
table 3 and the lower table 4 around an axis A in opposite
directions while supplying polishing solution such as pure water at
a predetermined supply amount between the polishing stones 5 and
the glass substrate 1. Thus, each of the glass substrates 1 slides
on the surfaces of the polishing stones 5, so that the both
surfaces of the glass substrate 1 are simultaneously polished.
[0044] The cerium oxide polishing stone is made of resin in which
cerium oxide powders are dispersed. For example, phenol resin,
epoxy resin, melamine resin, polyester resin, or urethane resin
that is used for a general polishing stone, or a mixture of two or
more of these resins can be used as the resin for the cerium oxide
polishing stone.
[0045] FIG. 5 is a plan view of the polishing machine 2 without the
upper table 3. Each of the carriers 6 retains five glass substrates
1 at maximum. A gear formed on an outer periphery of each of the
carriers 6 engages with a gear formed on an outer periphery of a
sun gear 7 and with an internal gear 8. With this configuration,
each of the carriers 6 rotates on its axis and moves along the
periphery of the sun gear 7, so that the both surfaces of each of
the glass substrates 1 retained by the carriers 6 are uniformly
polished.
[0046] In the main-surface precision-polishing process, the glass
substrates 1, which are made to be donut-shaped by the polishing
machine 2 in the previous process, are set for precision-polishing
the main surfaces of the glass substrates 1 until the glass
substrates 1 get a desirable thickness of 0.381 millimeter using a
hard-polyurethane polishing pad while supplying slurry including
colloidal silica. The main surface and end surface of the glass
substrate 1 that has been cleaned is visually checked. After the
visual check, a precise checking utilizing the light reflection,
scattering, and transmission is performed. It is confirmed that
there are no defects such as protrusion created by such as particle
adhesions or scratches on the main surfaces and the end surfaces of
the glass substrates 1.
[0047] After completion of the main-surface precision-polishing
process, surface roughness of each of the main surfaces of the
glass substrates 1 is measured by an atomic force microscope (AFM,
manufacturer: Shimadzu Corporation, model: SPM-9500J3). It is then
confirmed that the maximum surface roughness Rmax is 2.5 nanometers
and the average surface roughness Ra is 0.3 nanometer, which means
the surface is made to be super smooth. The values of the surface
roughness for the surface profile measured by the AFM are
calculated in accordance with Japanese Industrial Standards (JIS)
B0601.
[0048] Each of the glass substrates 1 is checked to confirm whether
the inner diameter is 7 millimeters, the outer diameter is 27.4
millimeters, and the thickness is 0.381 millimeter, which are
within the predetermined dimension range for a glass substrate used
for a 1.0-inch magnetic disk.
[0049] Subsequently, the surface roughness of an inner peripheral
end surface of the circular hole 1a of the glass substrate 1 was
measured. The measurement values were 0.4 micrometer for the
maximum surface roughness Rmax at the chamfered portion, 0.04
micrometer for the average surface roughness Ra at the chamfered
portion, 0.4 micrometer for the maximum surface roughness Rmax at
the side wall surface, and 0.05 micrometer for the average surface
roughness Ra at the side wall surface. Similarly, the surface
roughness of an outer peripheral end surface of the glass substrate
1 was measured. The measurement values were 0.04 micrometer for the
average surface roughness Ra at the chamfered portion, and 0.07
micrometer for the average surface roughness Ra at the side wall
surface. As the measurement values show, the inner peripheral end
surface is finished to be a mirror-like surface similarly to the
outer peripheral end surface.
[0050] It was also confirmed that each of the surfaces of the glass
substrates 1 had foreign objects or particles that may cause
thermal asperity thereon, and the inner peripheral end surface of
the circular hole 1a had no foreign objects or cracks thereon.
[0051] For the material of the glass substrate 1, a glass ceramic
such as an amorphous glass or a crystallized glass can be employed.
Especially, the amorphous glass is preferable from the viewpoint of
molding characteristics or machining characteristics. Other than
the amorphous aluminosilicate glass, the preferable glasses are,
for example, a soda-lime glass, a soda aluminosilicate glass, an
aluminoborosilicate glass, a borosilicate glass, an air-cooled or
liquid-cooled thermally toughened glass, and a chemically toughened
glass.
[0052] Nine samples No. 1 to No. 9 of the glass substrates 1 having
a surface roughness distribution of equal to or less than 0.1
nanometer, an outer diameter of 27.4 millimeters, an inner diameter
of 7 millimeters, and a thickness of 0.381 millimeter were produced
for comparison with the above manufacturing method. Then, nine
magnetic disks corresponding to the sample No. 1 to No. 9 were
produced with a sputter method under the condition of 0.27 pascal
under argon (Ar) atmosphere such that layers are sequentially
formed over each of the glass substrates 1 in the order of a
chromium titanium (CrTi) seed layer, a chromium (Cr) alloy
underlayer, a cobalt chromium platinum boron (CoCrPtB) alloy
magnetic layer, and a carbon protective coat. Each of the magnetic
disks was checked for its floating characteristics.
[0053] Each of the glass substrates 1 is manufactured such that the
magnitude of the waviness in the circumferential direction of the
main surface differs between the inner circumference and the outer
circumference by changing the pressure, the number of rotations,
and the roughness of the polishing stone 2 when rough-polishing the
glass substrate 1. The different waviness magnitudes between the
inner circumferential direction and the outer circumferential
direction of the main surface of the glass substrate 1 are realized
by changing the roughness of the polishing stone 2 in a radical
direction. The different surface roughnesses between the inner
circumference and the outer circumference are realized by changing
the condition of the slurry supply including colloidal silica when
fine-polishing each of the glass substrates 1. For example, this
can be achieved by supplying slurry from the outer circumferential
side of the glass substrate 1.
[0054] The measurement results of the average surface roughness Ra
and the average waviness Wa having a cycle of 300 micrometers to 5
millimeters in the circumferential direction of each of the glass
substrates 1 are as shown in FIGS. 7 and 8. As shown in FIG. 6, the
average surface roughness Ra1 of the inner circumference of the
glass substrate 1 is the average of the roughness in an annular
area E1 between 1 millimeter and 3 millimeters outward from the
inner periphery of the main surface of the glass substrate 1, and
the average surface roughness Ra2 of the outer circumference is the
average of the roughness in an annular area E2 between 1 millimeter
and 3 millimeters inward from the outer periphery of the glass
substrate 1. The average waviness Wa1 in the inner circumferential
direction is the average of the waviness in the annular area E1
having a cycle of 300 micrometers to 5 millimeters in the
circumferential direction, and the average waviness Wa2 in the
outer circumferential direction is the average of the waviness in
the annular area E2 having a cycle of 300 micrometers to 5
millimeters in the circumferential direction.
[0055] The values of the average surface roughness Ra1 and Ra2 in
the circumferential direction of each of the samples No. 1 to No. 9
of the glass substrates 1 and the deviation .sigma. (Ra) thereof
are measured by utilizing the AFM SPM-9500 J3 manufactured by
Shimadzu. The values of the average waviness Wa1 and Wa2 in the
circumferential direction and the deviation .sigma. (Wa) thereof
are measured by utilizing an interferometer (OPTIFLAT) for a
digital versatile disc (DVD) produced by Phase Shift Technology
Inc. The OPTIFLAT scans a predetermined area of a substrate surface
using a 680-nanometer-wavelength white light and calculates the
waviness by the interference pattern generated at a light combined
point at which a reflecting light from the substrate surface and a
reflecting light from the reference surface are combined.
[0056] The floating characteristics of the magnetic head with
respect to the magnetic disks corresponding to the samples No. 1 to
No. 9 of the glass substrates 1 were measured and the results are
shown in FIG. 9. As the floating characteristics, touchdown (TD)
characteristics and takeoff (TO) characteristics are measured using
a magnetic head used for an actual magnetic recording and
reproducing apparatus.
[0057] The TD characteristics are the measurement of the pressure
at the point that the magnetic head that has been stably floating
relative to a medium rotating at a constant number of rotations
under a constant environment touches down the medium by reducing
the pressure. The measurement is performed by an acoustic emission
sensor (an AE sensor) mounted on the magnetic head. The TO
characteristics are the measurement of the pressure at the point
that the magnetic head that has been touching a medium rotating at
a constant number of rotations under a constant environment floats
by increasing the pressure (no outputs from the AE sensor).
[0058] As shown in FIG. 9, the TO characteristics on the inner
circumferential side and the outer circumferential side of the
samples could be reduced excluding the samples No. 3, No. 4, No. 6,
and No. 9 down to about 0.6 atmospheric pressure. The TO
characteristics of the samples excluding the samples No. 1, No. 3,
No. 4, No. 6, and No. 9 were 0.65 atmosphere for the inner
circumferential side and 0.6 atmosphere for the outer
circumferential side. Particularly, the sample No. 1 showed an
unfavorable value of about 0.8 atmosphere for the inner
circumferential side. It is presumable that the sample No. 1 has a
small inner circumferential surface roughness and the magnetic head
is prone to adhere onto the surface, so that once the magnetic head
comes into contact with the magnetic disk, the magnetic head tends
to adhere to the magnetic disk resulting in unstable floating,
which may influence the TO characteristics. The sample No. 3 had
nearly the same surface roughness as those of the samples No. 2 and
No. 5, however, both the TD and TO characteristics of the inner
circumference showed the unfavorable value of 0.7 atmosphere.
According to an investigation of the cause, the waviness of the
inner circumference of the sample No. 3 exceeded 0.6 nanometer,
which was lager than those of the samples No. 2 and No. 5. Thus, it
is presumable that this large waviness has influenced the sample
No. 3. The sample No. 6 showed that the inner circumferential
surface roughness exceeded 0.8 nanometer and the difference between
the inner and the outer circumferential surface roughnesses
exceeded 0.2 nanometer. Therefore, both the TD and TO
characteristics were unfavorable. The sample No. 9 showed the
surface roughness level that was no problem, however, both the TD
and TO characteristics of the outer circumference showed the
unfavorable value of 0.7 atmosphere. The measurement of the
wavinesses of the inner and the outer circumferences showed that
the waviness difference between the outer circumference and the
inner circumference was as large as exceeding 0.2 nanometer. It is
therefore presumable that the waviness of the outer circumference
that was the largest in all the samples has influenced the sample
No. 9. For the samples No. 2, No. 5, No. 7, and No. 8, the TD
characteristics on the inner and outer circumferential sides and TO
characteristics on the outer circumferential side showed about 0.6
atmosphere, the TO characteristics on the inner circumferential
side showed about 0.65 atmosphere, and the difference between the
inner circumferential TD characteristics and the inner
circumferential TO characteristics showed an extremely favorable
value of about 0.05 atmosphere.
[0059] Based on the result of these TD and TO characteristics, the
glass substrates 1 of the samples No. 2, No. 5, No. 7, and No. 8
are considered to suppress adhesion of the magnetic head and
sufficiently prevent occurrence of fly stiction phenomenon. The
satisfactory conditions for the surface roughness and the waviness
of the glass substrate 1 can be derived referring to FIGS. 7 and 8,
which are at least Ra1.ltoreq.0.8 nanometer, 0
nanometer<Ra1-Ra2.ltoreq.0.2 nanometer, Wa1.ltoreq.0.6
nanometer, and 0 nanometer<Wa2-Wa1.ltoreq.0.2 nanometer.
[0060] In the glass substrate 1, only the annular areas E1 and E2
are stated, however, it is preferable that the value of the surface
roughness continuously or gradually increases from the outer
circumferential side to the inner circumferential side of the main
surface, and the average waviness value continuously or gradually
decreases from the outer circumferential side to the inner
circumferential side of the main surface. Furthermore, it is
preferable that the standard deviations .sigma. (Ra) and .sigma.
(Wa) of the fluctuation in the average surface roughness and the
average waviness in the circumferential direction of the glass
substrate 1 be respectively less than 0.05 nanometer.
[0061] The glass substrate 1 is preferably used for magnetic disks
for equal to or smaller than 1-inch HDDs.
[0062] Furthermore, the glass substrate 1 is preferably used for a
magnetic disk mounted on an HDD that starts up and stops by a
load/unload method.
[0063] In the present embodiment, wider range of the nanosized
waviness is controlled by at least satisfying Ra1.ltoreq.0.8
nanometer, 0 nanometer<Ra1-Ra2.ltoreq.0.2 nanometer,
Wa1.ltoreq.0.6 nanometer, and 0 nanometer<Wa2-Wa1.ltoreq.0.2
nanometer instead of conventional controlling of the average
surface roughness at a 5-micrometer.times.5-micrometer area of a
substrate. This waviness control can suppress adhesion of a head to
a disk, which may easily occur in a smaller-sized disk, and
sufficiently prevent occurrence of fly stiction.
[0064] According to an aspect of the present invention, even when a
hard disk is downsized for a downsized HDD mountable for a highly
portable device, occurrence of fly stiction can be sufficiently
prevented.
[0065] Although the invention has been described with respect to
specific embodiments for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art that fairly fall within the
basic teaching herein set forth.
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